For example, a conventional focusing error detecting equipment is disclosed as an optical head device in Japanese patent application laid-open No. 63-229640 (1988) first prior!. The optical system is shown in FIG. 1A. Light emitted from a light source 106 is transmitted through a diffraction grating beam splitter 107, converged upon a disk 109 by an objective lens 108. Then, the light reflected on the disk 109 is reversely passed through the same path, diffracted onto +1st-order diffracted light 107a and -1st-order diffracted light 107b by a diffraction grating beam splitter 107. The +1st-order diffracted light 107a is detected by a detector 110 while converging upon a focal point formed behind the detector 110, and the -1st-order diffracted light 107b is detected by a detector 111 while converging upon a focal point formed before the detector 111. As shown in FIG. 1B, the detector 110 is composed of light-receiving parts 110a, 110b and 110c which are parallel disposed, and the detector 111 is composed of light-receiving parts 111a, 111b and 111c which are parallel disposed. The +1st- order diffracted light 107a forms a beam spot 112a on the detector 110, and the -1st-order diffracted light forms a beam spot 112b on the detector 111.
The focal points of the +1st-order diffracted light 107a and -1st-order diffracted light 107b are moved before when the disk 109 goes away from the convergence point of the objective lens 108 due to a plane pitching etc. and are moved behind when the disk 109 comes close to the convergence point of the objective lens 108 due to a plane pitching etc. Namely, the beam spots 112a, 112b are reduced and enlarged, respectively when the disk 109 goes away from the convergence point of the objective lens 108 and, on the contrary, they are enlarged and reduced, respectively when the disk 109 comes close to the convergence point of the objective lens 108. Herein, provided that signals to be detected at light-receiving parts 110a to 110c are defined by signals T110a to T110c, respectively, a focusing error is detected by a focusing error signal G: EQU G=T110a-T110b+T110c-T111a+T111b-T111c.
Another conventional focusing error detecting device is disclosed as an optical head in T. Nagano et al., Proceedings of 12th Meeting on Lightwave Sensing Technology, LST 12-16, pp.103-109 second prior art!. The optical system is shown in FIG. 2. Light emitted from a laser diode chip 101 is reflected on a mirror 102, transmitted through a holographic optical element 103, converged upon an optical disk by a lens (not shown) which is disposed in the direction of a z axis in FIG. 2. Then, the light reflected on the optical disk is reversely passed through the same path, diffracted by the holographic optical element 103, detected by a photodiode chip 104. The laser diode chip 101 is fixed through a heat sink 105 on the photodiode chip 104. The holographic optical element 103 is so fabricated that, when the optical disk is located at the convergence point of the lens, the light diffracted by the holographic optical element is converged upon the photodiode chip 104. Light diffracted at a region 103a to be formed of the holographic optical element 103 is converged upon a division line 104a which divides a light-receiving part 104aa and a light-receiving part 104ab on the photodiode chip 104, and light diffracted at a region 103b to be formed of the holographic optical element 103 diagonally against the region 103a is converged upon a division line 104b which divides a light-receiving part 104ba and a light-receiving part 104bb on the photodiode chip 104.
FIGS. 3A to 3C are plan views showing beam spots to be formed on the photodiode chip 104. FIG. 3A shows the case that the optical disk is far away from the convergence point of the lens. FIG. 3B shows the case that the optical disk is located at the convergence point of the lens. FIG. 3C shows the case that the optical disk is located before the convergence point of the lens. Herein, provided that signals to be detected at light-receiving parts 104aa to 104bb are defined by signals R104aa to R104bb, respectively, a focusing error is detected by a focusing error signal E: EQU E=R104aa-R104ab+R104ba-R104bb.
Meanwhile, the focusing error signal E has to be zero when the optical disk is located at the convergence point of the lens. However, there occurs an offset due to a fabrication error, a long-term variation etc. in the focusing error detecting equipment. For example, the focus offset occurs when a difference between the optical path length from the holographic optical element 103 to the laser diode chip 101 and the distance from the holographic optical element 103 to the photodiode chip 104 is out of a design value. In such a case, the light diffracted by the holographic optical element 103 forms beam spots as shown in FIGS. 3A and 3C even when the optical disk is located at the convergence point of the lens. In the conventional focusing error detecting equipment, the focusing offset has been removed by rotating the holographic optical element 103 around the z axis when the focusing offset exceeds a tolerance in the optical head.
In the first prior art, there is a problem that the reflected light on the disk 109 is not available for an use other than the focusing error detection without using a further component. For example, the tracking error detection, which is indispensable to an optical head, cannot be achieved without using a three-beam grating etc. Thus, it must be costly due to the addition of the component. Further, there is another problem that the focusing offset cannot be removed even by rotating the diffraction grating beam splitter 107 when the focusing offset to exceed a tolerance is caused by a fabrication error, a long-term variation etc. in the optical head.
In the second prior art, there is a problem that the focusing error detection sensitivity is highly reduced when the mirror 102 is shifted to the direction of the y axis to the photodiode chip 104 due to a fabrication error, a long-term variation etc. in the optical head. The reason why the focusing error detection sensitivity is reduced is as follows: In general, the focusing error detection sensitivity is high when the distance from the cross point of the optical axis of the light diffracted by the holographic optical element 103 and the photodiode chip 104 to the division line 104a or 104b is short or it is low when the distance is long. Namely, when the mirror 102 is shifted to the direction of the y axis to the photodiode chip 104, the cross point of the optical axis of the light diffracted by the region 103a of the holographic optical element 103 and the photodiode chip 104 goes away from the division line 104a, and the cross point of the optical axis of the light diffracted by the region 103b of the holographic optical element 103 and the photodiode chip 104 goes away from the division line 104b. On the other hand, the reason why the reduction of the focusing error detection sensitivity is large is as follows: When the focusing error detection is conducted in a far field, the ratio between the amounts of light to be distributed to the light-receiving parts 104aa, 104ab and the ratio between the amounts of light to be distributed to the light-receiving parts 104ba, 104bb are not so varied even when the beam spot is shifted to some extent. However, in this focusing error detecting equipment, these ratios are highly varied because its focusing error detection is conducted in a near field.
Furthermore, in the second prior art, there is a further problem that the focusing error detection sensitivity is highly reduced when the holographic optical element 103 is rotated around the z axis on the photodiode chip 104 due to a fabrication error, a long-term variation etc. in the optical head, or when the holographic optical element 103 is rotated around the z axis to remove the focusing offset. The reason why the focusing error detection sensitivity is reduced is as follows: In general, the focusing error detection sensitivity is high when the distance from the cross point of the optical axis of the light diffracted by the holographic optical element 103 and the photodiode chip 104 to the division line 104a or 104b is short or it is low when the distance is long. Namely, when the holographic optical element 103 is rotated around the z axis on the photodiode chip 104, the cross point of the optical axis of the light diffracted by the region 103a of the holographic optical element 103 and the photodiode chip 104 goes away from the division line 104a, and the cross point of the optical axis of the light diffracted by the region 130b of the holographic optical element 103 and the photodiode chip 104 goes away from the division line 104b. On the other hand, the reason why the reduction of the focusing error detection sensitivity is large is as follows: When the focusing error detection is conducted in a far field, the ratio between the amounts of light to be distributed to the light-receiving parts 104aa, 104ab and the ratio between the amounts of light to be distributed to the light-receiving parts 104ba, 104bb are not so varied even when the beam spot is shifted to some extent. However, in this focusing error detecting equipment, these ratios are highly varied because its focusing error detection is conducted in a near field.